In order to separate in a grouped way the plutonium, neptunium, americium, curium and possibly the uranium from the lanthanides present in a dissolution solution of a used nuclear fuel, two methods have been recently proposed which use as an extractant, a malonamide such as N,N′-dimethyl-N,N′-dioctylhexyl-ethoxymalonamide (or DMDOHEMA) or a diglycolamide such as N,N,N′,N′-tetraoctyl-3-oxapentanediamide (or TODGA).
These methods are respectively described in the international PCT applications published under the Nos. WO 2007/118904 (reference [1]) and WO 2008/049807 (reference [2]).
It turns out that the extractants having donor oxygen atoms such as malonamides and diglycolamides do not allow extraction from an acid aqueous solution containing both actinides and lanthanides, of the actinides without extracting at the same time the lanthanides.
Consequently, the methods described in the aforementioned references first of all comprise a step aiming at co-extracting the actinides and lanthanides from the aqueous solution in which they are found, by means of an organic phase which contains the malonamide or the diglycolamide. This co-extraction step is followed by a step aiming at selectively stripping the actinides from the organic phase, which is achieved by means of a slightly acid aqueous phase, i.e. with a pH comprised between 2 and 3, and containing a complexing agent, for example a polyaminocarboxylic acid. The lanthanides are then retained in the organic phase either by the presence in this organic phase of an acid extractant of the phosphoric acid type (reference [1]) or by the presence in the slightly acid aqueous phase of nitrate ions (reference [2]). A step then follows aiming at stripping the lanthanides from the organic phase in order to recover these lanthanides in an aqueous phase capable of being subsequently subject to vitrification operations on the one hand and, at stripping the organic phase of radio-elements with view to its re-use on the other hand.
Now, in the perspective of developing novel methods for processing used nuclear fuels, it would be desirable to have extractants allowing grouped isolation of all the actinides present in dissolution solutions of used nuclear fuels. The methods for processing used nuclear fuels would notably be simplified and therefore less costly to apply.
Compounds are known which have a larger affinity for actinides and in particular for actinides(III), than for lanthanides.
These are nitrogen-containing polyaromatic compounds such as 2,2′:6′,2″-terpyridine and certain of its alkylated derivatives, 2,4,6-tri(2-pyridinyl)-1,3,5-triazine (or TPTZ), 2,6-bis(pyridin-2-yl)-4-amino-1,3,5-triazine (or ADPTZ) and 2,6-bis(1,2,4-triazinyl)pyridines, picolinamides, dipicolinamides and bipyridines with amide substitutions.
However, none of these compounds seems to be able to be used in an industrial process which would aim at grouped separation of the whole of the actinides present in dissolution solutions of used nuclear fuels from the lanthanides also present in these solutions, either because they are quite simply incapable of extracting alone the actinides from a strongly acid aqueous phase (which, for example, is the case of 2,2′:6′,2″-terpyridine and of its alkylated derivatives, of TPTZ, of ADPTZ and of picolinamides which are only capable of extracting them at a low acidity and in a synergistic mixture with another extractant, typically α-bromodecanoic acid), or because they have a too low loading capacity (this is, for example, the case of 2,6-bis(1,2,4-triazinyl)pyridines), or further because they require being in solution in a polar, halogenated and toxic diluent such as chloroform or meta-nitrotrifluorotoluene, and therefore may be hardly used in an industrial process (this is, for example, the case of dipicolinamides).
The Inventors therefore set themselves the goal of providing novel compounds which not only have larger affinity for actinides than for lanthanides but are further capable of extracting from a strongly aqueous acid solution the whole of the actinides present in this solution so that these compounds allow grouped isolation of all the actinides present in different degrees of oxidation in a dissolution solution of used nuclear fuels.
Further they set themselves the goal that the relevant compounds may be used in solution in a diluent capable of being used in an industrial process for processing used nuclear fuels.